(307d) Vortex Dynamics for High Levels of Polymer Drag Reduction: Quantitative Analysis Enabled By a New Vortex-Tracking Algorithm

Drag reduction (DR) caused by flexible polymers in wall-bounded turbulence has been studied for decades. A small dosage of polymers will significantly change flow structures in turbulence, and further reduce the energy losses due to the friction drag. Understanding the drag reduction phenomenon has significant practical implications for developing advanced flow control schemes. Starting from the pre-onset, the polymeric turbulence experiences several qualitative stages. In particular, previous experimental and numerical studies have shown that flow statistics become qualitatively changed once the high-extent drag reduction (HDR) regime is reached, suggesting the onset of a new drag reduction mechanism. In this study, this mechanism is investigated from the aspect of vortex dynamics. A new algorithm is developed for the tracking and extraction of vortex structures with complex three-dimensional configurations, which is applied to the turbulent flow fields of Newtonian and highly viscoelastic fluids. Vortex configuration and arrangement in these cases are compared. We found that polymers in the HDR regime tend to suppress the bursting phenomenon of turbulence by preventing the lifting process of near-wall vortices. This effectively suppresses vortex regeneration from the streak-instability mechanism and exposes the parent-offspring mechanism as the primary pathway for turbulence sustenance. The drastic change of the vortex regeneration mechanism qualitatively modifies the formation and distribution of vortices in the flow field and hence is responsible for the localization of turbulent structures and for the unique flow statistics observed at HDR.